Giant magnetostrictive materials (GMMs) are alloys of various rare earth and other metals with iron. Examples of such materials are described in U.S. Pat. No. 4,308,474, U.S. Pat. No. 4,374,665, U.S. Pat. No. 4,609,402 and U.S. Pat. No. 4,849,034. Early GMMs were developed principally for military applications, such as sonar, where considerations of power-consumption and cost were not the main design criteria. More recently, magnetostrictive actuators have been used for audio applications such as public address systems and portable personal audio systems (see for example WO02/076141) where low weight, cost and power consumption are important factors. The actuators are typically used to induce an audio signal into a surface, which can then radiate the signal, in effect acting as a loudspeaker. Actuators have been developed which contain small rods of GMM, for example of 10-15 mm in length and 2-5 mm diameter. These enable acceptable audio output to be achieved at a sufficiently low power consumption to permit satisfactory battery operation.
It is explained in WO02/076141 that it is desirable to achieve in a magnetostrictive actuator as near as possible a linear magnetic field through the length of the GMM rod. This may be achieved by biasing magnets at each end of the rod having a diameter substantially larger than that of the rod and preferably also by spacing the magnets from the ends of the rod by means of non-magnetic spacers. These requirements tend to limit the length of the rods which can be used, and while there is mention in WO02/076141 of the use of two or more pieces of GMM located end-to-end, in practice there may be little advantage in doing this, because the need to ensure a linear magnetic field imposes constraints on the length of GMM that can be used.
Since the change in length of the GMM rod in response to a change in magnetic field is a very small proportion of the length, the greater the length of the rod, the greater is the amplitude of vibration induced by the actuator into the surface. Thus, for improved audio quality at higher sound volume, it is desirable to use a longer rod.
These conflicting requirements limit the scope of designers to produce small, low-power magnetostrictive actuators that can produce high-quality sound.